A conventional power supply circuit is intended to be driven at a plurality of input-voltage sources which can have different voltages. The input-voltage sources cannot easily be interconnected. Therefore, a customary measure provides for the use of decoupling diodes, so that input-voltage source which has the higher voltage is connected to the further circuit, while the input-voltage source having the lower voltage is decoupled. The disadvantage of this known design approach is that the voltage difference at least occurring between one output of the power-supply circuit and the input-voltage source is increased by the forward voltage of one diode, in addition to the voltage difference occurring anyway in the power-supply circuit.
An object of the present invention is to specify a power-supply circuit which can be supplied from at least two different input-voltage sources, it is advantageous to provide as small a voltage difference as possible between the output of the power-supply circuit and the input-voltage source having the highest voltage.